Section B: Electricity

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Mains Electricity

Fuses are saftey devices - when the current flowing is too high, the fuse 'blows' cutting the circuit off

Circuit Breakers serve the same purpose but instead of replacing a fuse, circuit breakers can be easily reset

Double Insulation is when an appliance has a thick coating of an insulating material (plastic) to protect user from touching any electrical parts. Earth wire not needed

Live Wire - Provides path so electrical energy can flow (ac)

Neutral Wire - Completes the circuit

Earth Wire - Safety device

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Electrical Charge

Two ways of charging an object:

• Friction - when two uncharged objects are rubbed together, one of the objects may become negatively/positively charged depending on whether it gains/loses electrons (static electricity)
• Induction - sometimes if an object is charged it may charge an uncharged object by induction. If an object is negatively charged it may attradt the other object's positivity making the other object positively charged (the balloon and the wall)

Common Applications of Static Electricity:

• Electrostatic paint spraying
• Inkjet printers

Problems cause by Static Electricity:

• Aircrafts - increased amount of charge can cause explosions during refuelling if the plane is not earthed with a conductor on landing
• TV screens/computer monitors become charged and attract dust
• Clothing can become charged and deliver a small electric shock
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Current & Voltage in Circuits

SERIES CIRCUITS:

• One switch can turn all components on or off together
• If one component breaks, all other components will go off due to the gap in the circuit
• Voltage is "shared" (bulb brightness becomes dimmer)
• Current is the same at any point

PARALLEL CIRCUITS:

• Switches can turn on/off individual components in different parts of the circuit
• If one component breaks, only components in the same branch will be affected, others will work fine
• Voltage is the same at any point
• Current is "shared" (sum of the components)
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Electrical Resistance

ALL components in a circuit offer some resistance to the flow of charge

Using resistance:

• Fixed Resistors (can be used to protect bulbs)
• Variable Resistors (can be used to alter volume of a TV/radio or a dimmer switch)
• Thermistor - resistance changes with temperature:
• if temperatures are low, resistance is high
• used in fire alarms
• Light-Dependent Resistor (LDRs) - resistance changes with light
• in dark conditions, resistance is high
• used in photographic equipment (flash)

OHMS LAW - The current flowing through a conductor is directly proportional to the p.d across its ends (ONLY IF TEMPERATURE REMAINS CONSTANT)

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